In order to analyze a gas to be measured (referred to as “measurement target gas” hereinafter) flowing through a measurement target gas flow passage, if a part thereof is shunted to be measured, a flow rate in the measurement target gas flow passage is reduced in correspondence with a shunted flow rate, and this affects a control of the measurement target gas and its own and other measurements and may likely lead to a trouble in some cases.
For example, in Patent Literature 1, disclosed is a configuration that an exhaust gas discharged from an internal combustion engine is diluted with a dilution gas and the diluted exhaust gas (i.e., measurement target gas) is led to flow into a mini-tunnel (i.e., measurement target gas flow passage) and a part of the diluted exhaust gas is shunted to be led to a smoke particle measurement device. In this smoke particle measurement device, the smoke particles (also, referred to as “particulate matter” hereinafter) contained in the diluted exhaust gas are collected by a collecting filter so as to measure a mass thereof.
In this configuration, by locating a CVS device downstream of the mini-tunnel, the flow rate of the diluted exhaust gas flowing in the mini-tunnel is controlled to be constant and an introduction flow rate of the exhaust gas into the mini-tunnel can be controlled. This is because, by controlling the introduction flow rate of the dilution gas into the mini-tunnel, the introduction flow rate of the exhaust gas into the mini-tunnel is indirectly controlled.
By the way, the flow rate control mentioned above is implemented on the premise that the flow rate of the gas to be introduced into the mini-tunnel, i.e., measurement target gas flow passage is equal to a flow rate of the gas derived therefrom. Therefore, if the smoke particle measurement device takes in a part of the diluted exhaust gas from the mini-tunnel by shunting, there occurs an error in the introduction flow rate of the exhaust gas into the mini-tunnel accordingly, and this may also lead to occurrence of an error in a control of a dilution ratio of the exhaust gas and a measurement in the CVS device and the like.
Therefore, in Patent Literature 1, the diluted exhaust gas derived from the smoke particle measurement device after the smoke particles are collected is entirely led to reflow to the mini-tunnel so as to eliminate the error. In Patent Literature 1, as described in the second paragraph of column 2 at page 4 and FIG. 1 therein, prior to lead the diluted exhaust gas from the smoke particle measurement device to reflow, an appropriate flow rate of air is previously rendered flowing in a reflow passage, and when the diluted exhaust gas is rendered to flow back, a valve is switched to shut off the air. It seems that a large pressure fluctuation may not occur in the mini-tunnel at the time of starting the reflow.
However, as disclosed in Patent Literature 1, if it is configured that the measurement target gas subjected to a measurement can be returned as it is to the flow passage of the measurement target gas, the measurement errors etc. can be avoided by forming the reflow, but there may be nevertheless a case where the measurement target gas cannot be returned to the flow passage of the measurement target gas in such a case where the measurement target gas is diluted or absorbed according to a measurement device.
For example, in a particulate matter counting device for counting the number of the particulate matter, since the acquired measurement target gas is diluted within the device, the measurement target gas cannot be returned as it is. Conventionally, since the flow rate of the measurement target gas to be acquired by such a particulate matter counting device is not so large in amount compared to a flow rate of the measurement target gas flowing through the flow passage of the measurement target gas, if a small in amount, the error can be suppressed within a tolerance range even if the measurement target gas is not returned. However, in recent years, it is required to further improve a measurement accuracy and in the case where the flow rate of the measurement target gas flowing through the flow passage of the measurement target gas such as a micro-tunnel, it becomes impossible to suppress an error of such as a dilution ratio in an admissible range due to the fact that the measurement target gas cannot be returned to the source flow passage.
To give a specific numerical example, conventionally, the flow rate of the measurement target gas (diluted exhaust gas) acquired by, e.g., the particulate matter counting device is in a range of 0.1 to 0.5 L/min and the flow rate of the diluted exhaust gas flowing in the CVS device is set to 50 L/min. Then, there may occur an error in a dilution ratio of the diluted exhaust gas in a degree of 1% (=0.5/50) at the maximum. In recent years, however, since a tolerance error of the dilution ratio is required to be within 0.5% and in some cases within 0.1%, the error of 1% mentioned above exceeds the admissible range.
Citation List
Patent Literature
Patent Literature 1: JP-A-Heisei 03-218436